Rate of increase and critical amount of nuclear polyhedrosis virus in lepidopterous larvae estimated from survival time assay data with a birth-death model

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Abstract
A birth-death model developed for pathogens of vertebrates was used to estimate the in vivo rate of increase (α) and the doubling time (td) from survival time assay data. Host-pathogen combinations used in this study were twoAutographa californica nuclear polyhedrosis virus isolates inTrichoplusia ni andHeliothis zea NPV inH. zea. The α's, estimated as he negative reciprocal of the slope of the linearly decreasing section of the plot of median survival times against the logarithm of inoculum concentration, were calculated in two ways. First, simple regression was used to fit a line through the linearly decreasing part using data points selected by eye; secondly, a three-phase segmented linear regression model was used and α was estimated from the slope of the middle segment. Estimates of α (and td) were 0.338 (2.05), 0.274 (2.53) and 0.243 h−1 (2.85 h) using the simple regression method, and, 0.385 (1.80), 0.305 (2.27) and 0.223 h−1 (3.11 h) using the 3-phase segmented linear regression model for AcMNPV-1A, AcMNPV-HOB and HzSNPV, respectively. Although AcMNPV-HOB killed larvae faster (6 to 13 h) than AcMNPV-1A, it multiplied more slowly. Estimates for the critical number ranged from 4.8×109 to 4.5×1014 genome copies for HzSNPV inH. zea larvae and AcMNPV-1A inT. ni larvae, respectively. The significance of the calculated critical numbers is discussed.